Project Summary (Project 4) Genomic instability, which leads to genetic changes associated with progression of cancer to more aggressive phenotypes and ultimately the development of resistance to therapy, is a prominent feature of multiple myeloma (MM). Understanding genomic instability and its underlying mechanisms is, therefore, extremely important to develop therapeutic strategies which would suppress clonal evolution and its impact on genomic and clinical outcome. In previous funding period, we defined the mutational spectrum in MM at the time of initial diagnosis and found heterogeneity across samples, with largely distinct sets of chromosomal rearrangements and gene mutations present in individual patients. Importantly, serial sampling of patient MM cells revealed diverse patterns of clonal evolution, including linear evolution, differential clonal response, or branching evolution. We also identified at least two biologically distinct mutational signatures responsible for the majority of observed mutations, suggesting a pattern of parallel, divergent, or even convergent evolution. Interestingly, the number of mutations was the only factor correlating with overall as well as relapse free survival, thus highlighting the importance of understanding the mechanisms of genomic instability in MM. The clonal evolution which is probably driven by pressures from therapy, microenvironment, and inherent genomic and epigenomic mechanisms, leads to both clonal selection and formation of new clones. By deep sequencing of single gene IgH, we demonstrated that selection of very low frequency clone occurs following therapy, and that new clones may come up due to ongoing mutational changes. Consistent with our previous data which identified dysregulated homologous recombination (HR) as an important mechanism underlying genomic evolution, we have observed that patients with increased HR activity had poor event-free survival compared to the rest. We hypothesize that therapy affects both clonal selection and clonal evolution, driving the ultimate tumor genotype and phenotype that eventually emerges at relapse, suggesting that targeting underlying mechanisms of clonal evolution is necessary to achieve curative outcomes. To this end we will extend our ongoing investigations to investigate the impact of therapy on genomic instability and associated clonal selection, clonal evolution, and underlying mechanisms in MM (Sp Aim 1); identify mediators of genomic instability in MM (Sp Aim 2); and assess the ability of inhibitors of genomic instability to impact evolution of genomic changes in MM (Sp Aim 3). The proposed studies will further improve our understanding of genomic instability and progression of MM, identify novel drugs, and may facilitate the development of therapeutic strategies which would inhibit/reduce evolution and associated dismal outcome.